Arylnitrenes triplet

Arylnitrene triplet states have been observed by uv and esr when aryl azides are irradiated at low temperature in solid matrices. Flash photolysis of 1-azidoanthracene in ethanol at room temperature gives a short-lived intermediate (with a half-life of 3-10/nitrene generated by the matrix photolysis of the same azide at 77 When these solid matrices are allowed to warm up to room temperature the nitrene spectrum collapses and typical nitrene products vide supra) are obtained. The nature of the products formed by analysis of reaction mixtures is the main way used to adduce nitrene intermediacy on photolysis. 

The structure of the tar, formed in many azide decompositions, consists of polyazepine units,103 and arises by attack of arylamine, formed from triplet arylnitrene, on the azepine precursor 30 to give the l//-azcpin-2-amine 31, which acts as a nucleophile towards more azepine precursor to yield ultimately the polyazepine 95. 

As shown in Table 3, triplet lb is computed to be 25-26 kcal/mol lower in enthalpy than triplet lc.77 Table 3 also shows that radicals 8b and 8c, formed by adding a hydrogen atom to lb and lc, respectively, differ in enthalpy by only 1-3 kcal/mol. Therefore, the large enthalpy difference between 3lb and 3lc is not due to a difference between the abilities of the phenyl and pyridyl groups to stabilize an unpaired tt electron. Instead it must reflect an intrinsic enthalpy difference between arylnitrenes and arylcarbenes. Table 3 also shows that aniline (9b) and fl-picoline (9c) are also predicted to have very similar enthalpies, thus providing further evidence that the large enthalpy difference between lb and lc is, indeed, due to the fact that lb is a nitrene, while lc is a carbene. 

Huron and Platz recently smdied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1-2 ps. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in aryl-nitrenes and is comparable to a carbenic process. As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19. 

The dimerization of phenylnitrene to form azobenzene is a characteristic reaction of the triplet state of an arylnitrene. Thus, some intermediate formed along the reaction coordinate must undergo intersystem crossing to the triplet manifold. 

Assuming that singlet nitrene reacts with alkanes at near diffusion controlled rates allowed deduction of a rate constant of singlet-to-triplet nitrene intersystem crossing (ISC) of 2-8 X 10 s . This ISC rate is slower than in carbenes, but significantly faster than with arylnitrenes, which are discussed in a subsequent section. 

Triplet phenyl and triplet ort/zo-biphenylnitrene and other triplet arylnitrenes dimerize to form azo compounds (AZO). ... 

Fig. 5 Electronic absorption spectra of selected triplet arylnitrenes phenylnitrene (1, EPA), 2-fluorophenylnitrene (2, MCH), 2-cyanophenylnitrene (3, MCH) and 2.6-dicyanophenylnitrene (4, MCH) at 77 K.

Analysis of the data of Table 1 verifies the same finding that the shift of the near-UV absorption band of singlet arylnitrenes correlates with the shift of the intense near-UV absorption band of triplet nitrenes. Furthermore, the ort/io-substituents influence the absorption spectra of singlet and triplet phenylnitrenes more significantly than do para-substituents. 

For arylnitrenes only qualitative treatments have been carried out by Reiser et al. 30>. It was concluded that the ground state of arylnitrenes is also a triplet as expected from the experimental results. No quantum mechanical calculations have been reported for sulfonylnitrenes, but ESR-evidence indicates a triplet ground state also for these intermediates 47> (see below). 

Abstraction of hydrogen by the triplet nitrene to form primary amines is perhaps the most general reaction of arylnitrenes in solution. The source of hydrogen may be the solvent or the nitrene precursor. Although many arylazide ther-molyses were carried out under comparable conditions (see Table 5), there is no study available which deals with the relation between nitrene structure and the reaction paths which are possible. From the experimental data it can also be seen that the extent of hydrocarbon abstraction depends on the reaction medium, but no systematic study has as yet been reported. 

The stereochemistry of nitrene insertion into unactivated C—bonds has been studied using substituted cyclohexanes as substrates. For arylnitrenes which usually exhibit triplet reactivity, the reaction is nonspecific, but most other nitrenes undergo stereospecific C—insertion. For example, benzqylni-trene inserts selectively into the tertiary C—bond of toth cis and tra/u-1,4-dimethylcyclohexane with retention of configuration. Similarly with cis- and rra/is-l,2-dimethylcyclohexane as substrate, ethoxycarbonyl-, methatiesulfonyl- and cyano-nitrenes all insert with retention of configuration at the tertiary C—bond. 

The generation and reactions of arylnitrenes continue to attract attention. Evidence for a photoinitiated autocatalytic chain mechanism in the photodecomposition of phenyl azide has been reported, and the reaction of photochemically generated phenyl nitrene with oxygen has been reexamined. Irradiation of p-azidoaniline in aqueous solution yields triplet p-aminophenyl nitrene, which on reaction with water is converted into the highly reactive p-benzoquinone diimine. Both m- and p-nitrophenyl azides, on photoelimination of nitrogen at 77 K, afford the corresponding nitrenes, whereas o-nitrophenyl azide (93) is converted without the intermediacy of a nitrene into the benzofurazan (94). 4,4 -... 

Brauer have concluded that most of the previously reported red emission does not come from this molecule. Delayed fluorescence has been observed in the deactivation of highly excited triplet states (Skvortsov and Alfimov cf. McGimpsey and Scaiano). The photolysis of aryl azides, long used as a source of arylnitrenes, has been shown also to produce a transient dehydroazepine (Shields et al. Liang and Schuster). 

When, thermolysis of an aryl azide leads to an arylnitrene, the latter is generated in the singlet (182) state, which can be in equilibrium with the triplet (183). Electron spin resonance experiments have... 

Triplet phenyl nitrene and other triplet arylnitrenes dimerize to form azo compounds. 

Before moving on to the contribution of spectroscopic methods, we will note two early experiments. First, high dilution of phenylazide suppresses polymer formation and encourages the formation of the azo compound, implying that a singlet intermediate, such as benzazirine or ketenimine, can serve as a reservoir for generation of triplet arylnitrenes, which subsequently dimerize. 

They found that the triplet state is the ground state for all arylnitrenes studied. The lowest singlet state of nitrene 49 and its derivatives 56 to 58 was predicted to be the open-shell singlet. ° 3 The singlet-triplet energy gap AEgj was calculated to be 8.4 kcal/mol for 49 at the AMI level, and both... 

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